I'm not getting the attachment but this might help.
If your doing range as a circle with radius = r then:
if ((enemy.x-tower.x)**2+ (enemy.y-tower.y)**2)**0.5<=r: attack!!!
basically, you'd be using Pythagorean theorem to find the distance to check if it attacks or not.
Checking distances for every tower and every unit could become a bottle neck but would work.
If you have performance problems you could also incorporate a grid. When every unit moves it can do a check to see in what section of the map it is found in (ie, grid could have 10x10 tiles in each square) and you would only check the distance of creeps in the grids that are in range of the tower.
One thing to keep in mind would be that in some cases you may be able to stop the iteration as soon as you find someone who is attackable (if the tower attacks random creeps) otherwise you just compare the distances of the creeps and sort.
You may want to google for circle collision detection
-- Maranatha! --------------------------------- PFC aka Fezzik aka GAB
so, I wrote this tower defense game (zip archive attached) and I was wondering how I should go about implementing limited range (each tower can only shoot so far) in the code.